Sound deadening is equal parts art and science. The last rework I did on an SN95 resulted in a 4db(!) drop at cruise, and there was still a lot more work to be done. Even with the best products, poor implementation will yield garbage results and waste a ton of money.
Concentrate on these areas:
- Panel fit
- Air voids
- Air gaps
- Panel resonance
For panel fit (where 2 items come together, plastic on plastic, plastic on metal, whatever), I use thin/soft closed cell foam from the hardware store (window trim section). All you're trying to do is change how 2 items transfer energy to each other. Use it on any/all contact points (door panel - metal interface, where the shifter bezel attaches to the console, anything that could squeak).
Air voids. This is a great place for sound to echo/travel. When you stuff a void, you're trying to add resistance to convert wave lengths to heat (exactly like how a muffler works). Both SN's and foxes have HUGE air voids behind the 1/4 window plastic. I stuff those voids with denim insulation as it's flexible, easy to cut/stuff. Fill the
out of those voids - you want to take up all the air space. Home Depot used to carry a brand called Ultra Touch, and they had these little space filler rolls (R6.7) that were perfect for this. I do the same thing with door panels - stuff all voids with denim or use hard cell foam (trim to fit) if I can glue it permanently.
Air gaps. Make sure all your door/trunk rubbers are good. Make sure you're not leaking air around the mirrors. Seal your shifter boot (the rubber one) to the tunnel. In the doors, make sure your vapor barrier is 100% sealed. Hell, add a second one. Overcoming wind noise is HUGE for sound control.
Panel resonance. So this is a big one. And when you see people covering the entire floors/panels with elastomer, they are wasting $$ and adding weight that doesn't help their cause. I use this (
Vibro black - made in USA) as it's the heaviest by volume that I've found and the adhesive is fantastic. With this stuff, all you're trying to do is lower a panels resonant frequency - that's it. Take a small rubber mallet, tap sheet metal panels, and when you get one that vibrates, add some of that Vibro. You only need 20-30% panel coverage to change the resonant freq. This applies to floors, inside of doors (including backside of door skins), roof, any flat sheet metal. Pull the inner fenders and line the firewall - road noise vibrations enter here.
Sound blockage - I didn't cover this because it's really invasive and takes a lot of patience/work to set it up right or else you're wasting your time/$$/adding massive weight while still having sound leaks. So unless you're starting with a gutted car, don't bother going down this path.
There is a ton of opportunity under the back seat/spare tire well. Another small trick is the use 2 different length mufflers (same brand) as they will have slightly different resonate freqs and that will help with drone (constructive interference). Ford did this from the factory. If you can reduce the amount of sound you need to suppress, it makes the interior work more effective.